2.4 Scr0140: arithmetic operators  (Page 3/7)

As a binary operator, the minus character causes its right operand to be subtracted from its left operand. For example, the third statement belowsubtracts the variable y from the variable x and assigns the result of the subtraction to the variable z . After the third statement is executed, the variable z contains the value 1.

int x = 6;

int y = 5;

int z = x - y;

The minus character as a unary operator

As a unary operator, the minus character causes the algebraic sign of the right operand to be changed. For example, the second statement belowcauses a value of -5 to be assigned to (stored in) the variable x .

int y = 5;

int x = -y;

Binary operators use infix notation

To keep you abreast of the current jargon, binary operators in Scratch always use infix notation. This means that the operator appears between its operands.

Some other programming languages have unary operators that use prefix notation and postfix notation. For prefix notation, the operator appears before (to the left of) its operand. For postfix notation, the operator appears after (to the right of) its operand.

General behavior of an operator

As a result of performing the specified action, an operator can be said to return a value (or evaluate to a value) of a given type. The type of value returned depends on the operator and the type of the operands.

To evaluate to a value: To evaluate to a value means that after the action is performed, the operator and its operands are effectively replacedin the expression by the value that is returned.

Operator categories

There are many different kinds of operators. Therefore, the easiest way to study them is to divide them into categories such as the following

• arithmetic
• relational
• logical
• bitwise
• assignment

This module will concentrate on arithmetic operators. Future modules will deal with other kinds of operators.

Preview

In this module, I will present and explain a Scratch program named Arithmetic01 . This program illustrates the use of the following arithmetic operators (see Image 4 ):

• + (addition)
• - (subtraction)

Variables with the following names are created and displayed on the screen (see Image 5 ):

• LeftOperand - has a slider
• RightOperand - has a slider
• Sum
• Diff

In addition, a button is displayed on the screen.

The user adjusts the values of the LeftOperand and RightOperand variables with a pair of sliders. When the user clicks the button, the variablenamed Sum displays the sum of the values of the left and right operands. The variable named Diff displays the result of subtracting the right operand from the left operand.

Discussion and sample code

I'm going to walk you through the steps required to develop this program, being brief on those things that you already know about and being more verboseon the new material.

To begin with, Image 1 shows a reduced screen shot of the user interface.

Figure

Image 1. reduced screen shot of arithmetic01 in operation.

I realize that Image 1 lacks detail due to the requirement to reduce the size of the screen shot for publication in this format, but there are someaspects of Image 1 that are worth noting. I will go from left to right in Image 1 making comments.